Cellulosic substrates such as paper and cardboard (e.g., including corrugated fiberboard, paperboard, display board, or card stock) products encounter various environmental conditions based on their intended use. For example, cardboard is often used as packaging material for shipping and/or storing products and must provide a durable enclosure that protects its contents. Some such environmental conditions these packaging materials may face are water through rain, temperature variations which may promote condensation, flooding, snow, ice, frost, hail or any other form of moisture. Other products include disposable food service articles, which are commonly made from paper or paperboard. These cellulosic substrates also face moist environmental conditions, e.g., vapors and liquids from the foods and beverages they come in contact with. Water in its various forms may threaten a cellulosic substrate by degrading its chemical structure through hydrolysis and cleavage of the cellulose chains and/or breaking down its physical structure via irreversibly interfering with the hydrogen bonding between the chains, thus decreasing its performance in its intended use. When exposed to water, other aqueous fluids, or significant amounts of water vapor, items such as paper and cardboard may become soft, losing form-stability and becoming susceptible to puncture (e.g., during shipping of packaging materials or by cutlery such as knives and forks used on disposable food service articles).
Manufacturers may address the problem of the moisture-susceptibility of disposable food service articles by not using the disposable food service articles in moist environments. This approach avoids the problem simply by marketing their disposable food service articles for uses in which aqueous fluids or vapor are not present (e.g., dry or deep-fried items). However, this approach greatly limits the potential markets for these articles, since many food products (1) are aqueous (e.g., beverages, soups), (2) include an aqueous phase (e.g., thin sauces, vegetables heated in water), or (3) give off water vapor as they cool (e.g., rice and other starchy foods, hot sandwiches, etc.).
Another way of preserving cellulosic substrates is to prevent the interaction of water with the cellulosic substrate. For example, water-resistant coatings (e.g., polymeric water-proofing materials such as wax or polyethylene) may be applied to the surfaces of the cellulosic substrates to prevent water from contacting the cellulosic substrates directly. This approach essentially forms a laminated structure in which a water-sensitive core is sandwiched between layers of a water-resistant material. Many coatings, however, are costly to obtain and difficult to apply, thus increasing manufacturing cost and complexity and reducing the percentage of acceptable finished products. Furthermore, coatings can degrade or become mechanically compromised and become less effective over time. Coatings also have the inherent weakness of poorly treated substrate edges. Even if the edges can be treated to impart hydrophobicity to the entire substrate, any rips, tears, wrinkles, or folds in the treated substrate can result in the exposure of non-treated surfaces that are easily wetted and can allow wicking of water into the bulk of the substrate.
Furthermore, certain coatings and other known hydrophobing treatments for cellulosic substrates may also render the substrates not biodegradable. Therefore, it would be desirable to provide a method for rendering cellulosic substrates hydrophobic as well as maintaining their biodegradability.
It would also be desirable to conduct the treatment method in a way that ensures not just that the substrate is rendered hydrophobic, but also the efficient operation of the process. For example, if a liquid mixture of halosilane with a volatile solvent is used to saturate a substrate such as paper, when the solvent is evaporated the paper may be rendered hydrophobic. However, a significant portion of the halosilane evaporates with the solvent in known processes. In a commercial operation this stream containing solvent and halosilane must be processed in some way.
One way to process the stream would be to condense the solvent and halosilane. Unfortunately, because the evaporation of the solvent from the paper removes some amount of water from the paper, condensing the mixed vapor causes water to condense as well. The condensed water reacts quickly with the condensed halosilane forming a siloxane plus hydrogen halide. When an organohalosilane, such as a monoorgano, trihalo silane condenses with water present, it forms solid by-products, which must be separated from the process and discarded. Thus practicing a liquid treatment method requires the handing of a by-product stream that includes a volatile solvent and a solid or even gelatinous mixture which includes a hydrogen halide.
Vapor treating methods have also been proposed. However when treating paper with vaporized halosilane using a known process, there is still a by-product stream to handle. The by-product stream includes solvent and the portion of the halosilane which did not react into the paper during treating.
There is a commercial need for a method that enables substrates such as paper to be treated using a halosilane with a large fraction of the halosilane remaining in the paper and not requiring treatment as a by-product stream.